The present invention is directed to a photomask as well as to a method for the manufacture of the photomask.
In a flaw density measurement on a photomask, the object to be measured is irradiated with light. The interaction of the light with the photomask is then analyzed. A photomask normally contains a plurality of chip fields. Each chip field is usually subdivided into individual cells. During the measurement, a gray scale value that occurs from the intensity of the analyzed light is allocated to every cell. The measured gray scale values of the cells of a chip field are then compared to a database, which would be a die or semiconductor to database inspection, and the database contains a corresponding design realized in the chip field. It is also possible to compare the corresponding gray scale values of an identical chip field on the same photomask, which would be a die-to-die inspection.
Two methods for the inspection of photomasks are usually implemented, and these are a bright field inspection and a dark field inspection. In the bright field inspection, the illumination of the photomask occurs perpendicularly from above. The light that is perpendicularly reflected upward or the light that penetrates through the photomask is analyzed. In the dark field inspection, the illumination of the photomask occurs perpendicularly from above or given lateral incidence. The light reflected upward in various spatial directions is then analyzed.
In addition to containing the information of a potentially present flaw, the intensity of the reflected light always mainly contains parts that arise due to dispersion or reflection of the light at the structure realized in the chip field. These parts are, in turn, dependent on the design of the respective chip. The intensity of these parts and their spatial distribution significantly determine the obtainable precision of the measurement.
Inspections of different designs or products thereof never have sensitivities that are comparable to one another. This is not a problem in the inspection of mass-produced products in semiconductor wafer manufacture. So many identical wafers are always measured, that a stable trend will occur. A process or system problem then shows up as a violation of a defined specification.
This trend, however, practically never occurs in the fabrication of photomasks, since only very small piece numbers, usually less than 10, of each photomask are fabricated, and the sensitivities of the inspection results of different groups of masks are not comparable due to the different designs. When the stability of a manufacturing tool for the manufacture of a photomask is nonetheless to be monitored, a way must be found for comparing inspection results of various products and designs to one another.
In the inspection of surfaces of semiconductor wafers, it is known to provide a wafer having standard artificial defects to be used for calibrating the inspection device. An example of this is disclosed in U.S. Pat. No. 5,691,812, whose disclosure is incorporated herein by reference thereto.